[Audit][High] V2 Generator: incorrect underwater detection marks underground stone as submerged

[MichaelFisher1997]

🔍 Module Scanned

(automated audit scan)

📝 Summary

In , the variable is computed incorrectly at line 379. The condition marks ANY stone block below sea_level-1 as "underwater" even when it's inside a mountain that rises above sea level, causing wrong surface/filler blocks to be placed.

📍 Location

• File:
• Function/Scope:

🔴 Severity: High

• High: Memory leaks, race conditions, incorrect rendering, broken features

💥 Impact

When terrain extends below sea level (e.g., a mountain from y=80 down to y=50), stone blocks at y=62 will be treated as "underwater" and get sand/gravel surface instead of normal surface blocks, even though the mountain top is above water. This causes visible artifacts: underwater terrain textures appearing on hillsides above water line, and incorrect filler block composition in underground sections of above-sea-level terrain.

🔎 Evidence

The loop iterates top-to-bottom. The variable already tracks what block is directly above the current position. When , there is genuinely water above and the block IS underwater. But is a global height check that doesn't account for mountains that pierce the water surface.

Example bug scenario:

• Mountain with surface at y=80, extending underground to y=50
• When filling at y=62 (inside mountain, but below 64): = = - but at y=60: = , so stone at y=60 gets "underwater" treatment even though might be (not water)

🛠️ Proposed Fix

Remove the incorrect height-based fallback. The check is sufficient:

The logic should ONLY use because:

1. The loop processes top-to-bottom, so correctly reflects what sits above the current block
2. If , the current block is genuinely submerged
3. If or (etc), there's no water above regardless of global height

The original was likely trying to handle floating underwater caves or rock spires, but the correct solution is to rely on the per-block tracking.

✅ Acceptance Criteria

• Stone blocks inside above-sea-level mountains below y=sea_level-1 get correct surface (grass/dirt) not sand/gravel
• Stone blocks genuinely underwater still get correct underwater surface (sand/gravel)
• Zig 0.16.0 + SDL3 Dev Environment
  Compiler: 0.16.0
  assets/shaders/vulkan/water.frag
  assets/shaders/vulkan/culling.comp
  assets/shaders/vulkan/ui_tex.vert
  assets/shaders/vulkan/shadow.vert
  assets/shaders/vulkan/debug_shadow.frag
  assets/shaders/vulkan/debug_shadow.vert
  assets/shaders/vulkan/mesh.comp
  assets/shaders/vulkan/water.vert
  assets/shaders/vulkan/ssao.vert
  assets/shaders/vulkan/terrain.vert
  assets/shaders/vulkan/ui_tex.frag
  assets/shaders/vulkan/terrain.frag
  assets/shaders/vulkan/depth_pyramid.comp
  assets/shaders/vulkan/ssao_blur.frag
  assets/shaders/vulkan/ui.frag
  assets/shaders/vulkan/shadow.frag
  assets/shaders/vulkan/sky.frag
  assets/shaders/vulkan/bloom_downsample.vert
  assets/shaders/vulkan/bloom_upsample.vert
  assets/shaders/vulkan/debug_shadow.vert
  assets/shaders/vulkan/fxaa.vert
  assets/shaders/vulkan/post_process.vert
  assets/shaders/vulkan/shadow.vert
  assets/shaders/vulkan/sky.vert
  assets/shaders/vulkan/ssao.vert
  assets/shaders/vulkan/taa.vert
  assets/shaders/vulkan/terrain.vert
  assets/shaders/vulkan/ui.vert
  assets/shaders/vulkan/ui_tex.vert
  assets/shaders/vulkan/water.vert
  assets/shaders/vulkan/bloom_downsample.frag
  assets/shaders/vulkan/bloom_upsample.frag
  assets/shaders/vulkan/debug_shadow.frag
  assets/shaders/vulkan/fxaa.frag
  assets/shaders/vulkan/g_pass.frag
  assets/shaders/vulkan/post_process.frag
  assets/shaders/vulkan/shadow.frag
  assets/shaders/vulkan/sky.frag
  assets/shaders/vulkan/ssao.frag
  assets/shaders/vulkan/ssao_blur.frag
  assets/shaders/vulkan/taa.frag
  assets/shaders/vulkan/terrain.frag
  assets/shaders/vulkan/terrain_debug.frag
  assets/shaders/vulkan/ui.frag
  assets/shaders/vulkan/ui_tex.frag
  assets/shaders/vulkan/water.frag
  assets/shaders/vulkan/culling.comp
  assets/shaders/vulkan/depth_pyramid.comp
  assets/shaders/vulkan/lpv_inject.comp
  assets/shaders/vulkan/lpv_propagate.comp
  assets/shaders/vulkan/mesh.comp
  assets/shaders/vulkan/taa.vert
  assets/shaders/vulkan/g_pass.frag
  assets/shaders/vulkan/sky.vert
  assets/shaders/vulkan/taa.frag
  assets/shaders/vulkan/ssao.frag
  assets/shaders/vulkan/lpv_propagate.comp
  assets/shaders/vulkan/lpv_inject.comp
  assets/shaders/vulkan/ui.vert passes
• Visual verification: mountains rising from ocean show correct surface textures at all heights

📚 References

• Luanti v7 mapgen that this is ported from: https://github.com/luanti-org/luanti/blob/master/src/mapgen/mapgen_v7.cpp
• Issue Phase 5: Cave system refactor #108 cave system references in

[github-actions]

The bug is confirmed at modules/worldgen-overworld-v2/src/root.zig:379.

Root Cause Analysis:

// Line 379
const underwater = above == .water or y < self.params.sea_level - 1;

The y < self.params.sea_level - 1 fallback is incorrect because it ignores whether the current column's terrain surface is actually above sea level. The loop processes top-to-bottom, so above already correctly reflects what sits directly above each block.

Bug scenario: Mountain surface at y=80, extending underground to y=50. When filling at y=62 (inside mountain, below sea level 64, but column's terrain_height=80):

• above may be .stone (not water)
• BUT the height check y < sea_level - 1 (i.e., 62 < 63) evaluates to true
• So stone gets "underwater" treatment even though no water exists above

Proposed fix: Remove the height-based fallback, keeping only above == .water:

const underwater = above == .water;

This is correct because:

1. Loop iterates top-to-bottom, so above is always accurate
2. If above == .water, the block is genuinely submerged
3. If above is stone/air/etc., there's no water above regardless of global height

The original height check was likely intended to handle floating underwater caves/rock spires, but the per-block above tracking is the correct solution for that case as well.

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